No they dont loook like prologue cars they look much worser but still prologue cars looked fantastic.
The premium cars have over 500k polygons per car, the standards have 200-250k, the cars from prologue have the same.
the cars have been boosted alot since prologue, that was 3 years ago.
You can just google Gran Turismo 5 Polygon count, or Gran Turismo 5 Prologue polygon count/Gran Turismo 5 Premium polygon count to find out this information.
One thing to understand is... those polygons aren't applied all in one area. the standards may have better wheel motion,detail in small area's than the ones from prologue, but yes some standards do look alot better than the others, like the merc/veyron/land rover are the best standards I've seen so far, others may have 100k less polygons, you can see it in some of them. but they may range in polygon count like they did in previous GT games.
Gran Turismo 3, PS2, 2001
Cars - ~2,000-4,000 polygons
Gran Turismo 4, PS2, 2004
Cars - ~2,000-5,000 polygons